The United Nations estimates that 16,000 individuals worldwide were infected with HIV each day in 1998; by year's end, more than 33 million people were living with the virus. At least 95% of all people infected with HIV live in regions with limited resources such as sub-Saharan Africa, where recently developed antiretroviral treatments are neither affordable nor widely available.
And while highly active antiretroviral therapy (HAART) has provided therapeutic benefit to those who can obtain and tolerate it, HAART has not cured a single individual. Nor has its use since 1996 been proven to reduce the rate of HIV transmission, apart from perinatal (mother-to-infant) transmission reduced through the use of nevirapine (Viramune) monotherapy, AZT (Retrovir) monotherapy, and AZT/3TC (Epivir) combination therapy. According to the Centers for Disease Control and Prevention (CDC), approximately 40,000 new infections occur each year in the U.S. -- a figure that has held steady for the past decade.
Many experts feel that a sustained decrease in HIV transmission and containment of the AIDS epidemic can be achieved only with widespread use of a safe, effective vaccine (though safer sexual behaviors and the use of clean needles will continue to be important components of AIDS prevention since no vaccine is 100% effective). Yet while substantial scientific effort, the diligent work of advocates, and a considerable outlay in funds have gone into creating HAART, HIV vaccine technologies have not been pursued with the same intensity in the pharmaceutical or biotechnology industries.
For these reasons, and in light of the needs of increasing numbers of HIV positive people, the general emphasis for many years has been on treating those already infected with the virus and advocating prevention by education and behavior modification. This is now changing, due in part to a growing awareness of the benefits of a vaccine coupled with an increased commitment by the government, private organizations, and philanthropists. Yet even though interest in vaccines seems to be gaining momentum, the development of a comprehensively effective vaccine strategy must overcome significant scientific obstacles.
Persistent viral replication, a hallmark of HIV, results in billions of new virus particles (virions) per day. The virus itself is highly unstable: mutant (genetically different) varieties of HIV proliferate rapidly during infection, resulting in the constant generation of virus that can escape recognition -- and thus regulation -- by the immune system. These mutant viral strains overwhelm the immune system's efforts to combat HIV with cytotoxic T-lymphocytes (CTLs, or killer T-cells) and antibodies, and leads to immunodeficiency and disease in almost all cases, at least without HAART. Protein structures on the HIV envelope (outer layer) are encrusted with sugar molecules, which further enables the virus's ability to elude the immune system.
Researchers have tried to adapt standard immunization methods or develop new ones to cope with the unique difficulties associated with the virus. A wide variety of approaches has already been evaluated: between 1988 and 1998, the AIDS Vaccine Evaluation Group (AVEG) of the National Institute of Allergy and Infectious Diseases (NIAID) conducted 47 studies using 22 different concepts. Although 36 vaccine candidates are currently in clinical trials in the U.S., most are in very preliminary stages. Shepherding research from the early stages into phase III efficacy trials has proven to be the most significant challenge for researchers.
Due to safety and practical concerns, early research done on vaccine candidates continues to rely on in vitro (test tube) models of HIV. The virus behaves quite differently, however, in a laboratory setting compared with a living human body. Promising vaccine approaches based on in vitro data therefore often fail in studies further along in the research pipeline. In fact, few of the vaccine approaches tested so far in phase I human trials have induced antibodies able to sufficiently neutralize primary isolates of HIV (Tat toxoid vaccines are a notable exception; see below).
Using animal models to evaluate experimental therapies in vivo (in a live animal) before testing them in humans also poses problems. To date, no perfect animal model has been found for replicating the effects of HIV in humans. Early studies in rabbits and other rodents (such as the severe combined immune deficiency [SCID]-hu mouse model) yield important data regarding immune responses to investigational vaccines and their relative safety, but transposing such animal data directly to human models can be extremely risky, especially when vaccine doses and concentrations are as yet unestablished.The so-called great apes, such as chimpanzees, are also potentially ideal research subjects since they are the only species besides humans susceptible to infection by HIV-1.
Yet most chimpanzee studies have been disappointing; these monkeys display a very low level of viral replication when infected with primary isolates of HIV, which furthermore induce disease in chimpanzees only after many years (similar to the human AIDS incubation period). Asian macaques can display AIDS-like symptoms rapidly when infected with virulent strains of simian immunodeficiency virus (SIV, a relative of HIV) or chimeric SHIV, a hybrid virus composed of an SIV core surrounded by HIV envelope proteins. Research using macaques is ongoing. Until a universally accepted approach for testing vaccine strategies in nonhuman primates is specified, few vaccine candidates will be considered for large-scale phase III efficacy trials in humans.
Researchers are trying to determine the optimal coordination of both arms of the immune system and the cytokines, or signaling molecules, that orchestrate them. At first it was believed that an HIV vaccine should provide sterilizing immunity (complete protection from infection), implying a key role for neutralizing antibodies. Because sterilizing immunity does not appear to be either feasible or (as with measles and hepatitis B) perhaps even necessary to prevent disease, however, vigorous production of CTLs -- along with Th1 helper cells, which are a particular subset of CD4 cells that help the CTLs work properly -- may be crucial to clear the virus or at least keep the viral load extremely low for an extended period of time.
The need for a strong cell-mediated response has been documented by studies of exposed but uninfected persons. Examples include African sex workers, selected long-term nonprogressors (LTNPs, or persons who do not exhibit immune dysfunction despite being infected with HIV for many years), health-care workers exposed to HIV but uninfected after a needlestick injury, and uninfected infants born to HIV positive mothers. Assays that measure CTL responses must be improved for a better understanding of their role in immune protection.
A successful vaccine strategy must also provide long-term protection against all clades (strains or subtypes) of HIV, especially those subtypes prevalent in sub-Saharan Africa (clades A, C, and D) and Southeast Asia (clade E). Most current HIV vaccine candidates have been tested only against laboratory strains of clade B virus, the subtype prevalent in Western Europe, Australia, Japan, and the Americas. Provoking immune responses at mucosal sites is crucial, as the most common mode of HIV transmission worldwide is by exposure to the virus at vaginal, rectal, and other gastrointestinal sites (including transmission to infants through breast milk).
Seroconversion induced by a vaccine is not identical to an infection with HIV. Although vaccinated individuals can be identified using the Western blot confirmatory test, HIV vaccines can produce false-positive results on the initial HIV antibody screening test (ELISA) and cause unnecessary trauma to the vaccinee. Preventive vaccination strategies therefore should allow vaccinees to be distinguished from HIV-infected persons. At present, those who have been inoculated usually are given a letter or card designating them as experimental vaccine recipients.
Treatment vaccines may work synergistically with HAART to control viral replication. Fred Valentine, M.D., of New York University, and colleagues reported on a 43-subject, 32-week, double-blind clinical trial of Remune (inactivated HIV) at this past year's 12th World AIDS Conference in Geneva, Switzerland. Remune is a treatment vaccine also known as the HIV-1 Immunogen or the Salk HIV vaccine. Persons treated with HAART and then immunized with Remune displayed a strong lymphoproliferative (increased production of CD4 and CD8 cells) response to a specific HIV core antigen, p24. A similarly robust immune response to p24 antigen also is seen in LTNPs and persons whose HIV viral load has been suppressed with HAART during acute infection (see also "Conference Coverage," BETA, October 1998). It is hoped that once viral load has been suppressed by HAART, an immune-based therapy, or vaccine such as Remune, can control further HIV replication without HAART.
Results from recent trials have been mixed. On May 17, 1999, an independent Data and Safety Monitoring Board (DSMB) advised the Immune Response Corporation of Carlsbad, CA, to halt its 2,500-subject, placebo-controlled phase III trial of Remune because differences in clinical endpoints (outcomes such as progression to AIDS or death) were not observed between those who added the vaccine to their anti-HIV therapy and those who did not. HAART became the standard of care after the trial began; its use may have masked additional benefits rendered by the vaccine. Studies of 250 of the participants showed that those who took Remune displayed a greater reduction in their HIV viral loads and an increase in lymphoproliferative responses at 48 and 96 weeks. Clinical endpoints thus may have been misleading indicators of the long-term efficacy of combining HAART with Remune; to gather more pertinent data, phase II trials to analyze secondary endpoints (surrogate markers such as viral load and lymphocyte production) are currently being done in Argentina, Spain, and Thailand.
In addition, a pivotal phase III trial in North America, Europe, and Australia has recently been announced. This 48-week study will try to determine whether adding Remune to a specific HAART regimen (nelfinavir [Viracept] and AZT/3TC [Combivir]) delays the onset of virologic failure in treatment-naive individuals. Failure in this context means that viral load either does not decrease to below 50 copies/mL or rebounds to greater than 50 copies/mL. For more information, call 877-858-7783.
A much smaller, 40-subject Remune trial for HIV positive persons already taking HAART has also started enrolling at the University of Alabama at Birmingham. Participants will receive injections of Remune at weeks 0, 12, and 24, then stop their antiretroviral therapy at week 28 to determine if their viral loads remain suppressed (i.e., below 50 copies/mL) without HAART. For more information, call 205-975-9127.
In Europe, a phase II trial conducted by the European Multinational IMMUNO AIDS Vaccine Study Group showed that a therapeutic vaccine based on a recombinant, or genetically engineered, surface glycoprotein (rgp160) conferred no clinical benefits in trial volunteers. According to a report in the August 20, 1999 issue of AIDS, 96 treatment-naive volunteers with CD4 cell counts greater than 500 cells/mm3 and 112 treatment-experienced volunteers (51 of whom were on treatment with one or two nucleoside analog [NRTI] drugs) with CD4 cell counts ranging from 200-500 cells/mm3 participated in the trial. Frank D. Goebel, M.D., of the University of Munich, and colleagues administered monthly injections of either the rgp160 vaccine or placebo (an inactive substance) to the participants for six months. Booster immunizations (supplementary vaccine doses) or placebo were subsequently given after 15, 18, and 21 months.
At the two-year follow-up, Dr. Goebel's group found that the vaccine had no effect on the participants' viral RNA or proviral DNA levels despite the initiation of rgp-160-specific lymphoproliferative responses. Because the rgp160 vaccine had no effect on the natural history of HIV infection among trial subjects, the Study Group concluded that this particular vaccine is not highly efficacious and should no longer be pursued.
Immunologists are now exploring ways of combining DNA vaccines with adjuvants (immune stimulators) to elicit immune responses strong enough to protect the body from future infection and optimize the ratio of cellular to humoral responses. For example, incorporating genes for certain cytokines (such as interleukin 2 [IL-2] and IL-12) and chemokines (such as MIP-1 alpha) into DNA plasmids has been shown to boost CTL responses to HIV. (Chemokines are cytokines that attract and activate lymphocytes. For more information on chemokines and vaccines, see "Chemokines and HIV," BETA, March 1997.) DNA vaccine constructs can be further engineered to carry genes from different strains of HIV, potentially providing immunity against several clades at once. The ability to manipulate and customize DNA vaccine therapies is one of their most attractive features. DNA vaccines are believed to be unable to cause HIV infection -- an obvious safety concern with most candidate vaccines -- since they lack the genes necessary for viral replication. Moreover, DNA vaccines are easy to design and generate in large quantities, and therefore should be relatively inexpensive to produce and distribute throughout the world (see also "DNA and Other Technologies for HIV Vaccines," BETA, April 1998).
In July, Dr. Emilio Emini of Merck & Co., based in Whitehouse Station, NJ, confirmed reports that Merck was planning to begin human trials of two experimental vaccines, including a DNA-based approach aimed at eliciting significant amounts of CTLs. John Shriver, also of Merck, explained that their approach involves genetically altering HIV genes to "humanize" their sequences, (i.e., by changing some of the DNA codons -- specific nucleotide sequences -- to those found most commonly in humans). Merck's other vaccine will be a live vector-based approach of unknown composition that will probably serve as a booster dose for the DNA model. The trials should begin enrolling by the end of this year.
Researchers are investigating the potential of avian (bird) pox viruses, such as attenuated (weakened) canarypox, which have a limited ability to replicate in the body. It has not yet been determined if such vectors will provide a strong, reliable, and long-lasting CTL response. In July, Dr. Robert Belshe of the St. Louis University School of Medicine reported on a multicenter, phase II trial of Pasteur Mérieux Connaught's canarypox vector ALVAC vCP-205 combined with Chiron Vaccine's recombinant glycoprotein 120 (known as SF-2 rgp120). Over 400 participants received either the vector plus rgp120, the vector plus saline, or placebo plus saline at trial start and at intervals of one, three, and six months.
Not surprisingly, the neutralizing antibody response was greatest in those who received the vector plus rgp120; however, a CTL response was demonstrated in only 30% of vaccinated subjects at any given time. Such a low CTL response level has raised questions about the utility of this canarypox. Other vectors being evaluated include adenovirus, modified vaccinia Ankara (MVA), Venezuelan equine encephalitis virus (VEE), other alpha viruses (including Semliki Forest virus), and enteric (intestinal) bacteria such as salmonella.
Most subunit vaccines are monovalent, meaning they have only one molecule against which an immune response can be directed. Antibodies generated by monovalent subunit vaccines have not been able to neutralize primary isolates of HIV efficiently, nor can they neutralize more than a limited number of strains. Development of a bivalent vaccine -- for instance, using rgp120 from one strain of HIV and combining it with rgp120 from another -- has resulted in an antibody response of much greater breadth.
The only phase III efficacy trials of a prophylactic HIV vaccine to date involve a bivalent rgp120 approach known as AIDSVAX, developed by Donald P. Francis, M.D., of VaxGen, Inc., based in Brisbane, CA. One study -- with 60 trial sites in North America and one in Amsterdam -- is enrolling 5,000 volunteers who will be randomized to receive vaccine (based on clade B viruses) or placebo in a 2:1 ratio and monitored for three years.
The other study -- using a bivalent vaccine based on clade E viruses -- is ongoing at 17 trial sites in Bangkok. Many people question the value of testing gp120 vaccines like AIDSVAX in phase III trials because they do not induce CTL production and, even with a bivalent structure, they may not produce antibodies capable of neutralizing adequate amounts of primary HIV isolates. Others feel encouraged that at least one prophylactic vaccine candidate has reached a relatively advanced stage of development despite its apparent drawbacks.
This unfavorable outcome was observed in a study conducted by researchers at the Dana-Farber Cancer Institute in Boston involving 16 adult macaques. As reported in the February 1999 issue of Nature Medicine, four monkeys in the trial had HIV in their blood, one displayed some immune dysfunction, and one died of AIDS; all had been inoculated with a weakened strain of SIV that lacked three different genetic elements (nef, vpr, and the Nef-responsive element, or NRE) involved in viral replication.
Recent news from Australia has dealt another blow to the prospect of using a live, attenuated HIV vaccine in humans. A report in the June 3, 1999 issue of the New England Journal of Medicine indicated that three Australians infected with a strain of HIV lacking a functioning nef gene were experiencing immune system damage, including detectable viral loads and declining CD4 cell counts. The three were among a group of nine people -- including a blood donor and eight people who received transfusions of his blood before 1985 -- infected with the same attenuated HIV strain. Known as the Sydney Blood Bank Cohort, the remaining six members of this group appeared to be LTNPs (three have died of causes unrelated to HIV infection). The fact that some of them are now showing signs of disease progression indicates that attenuated HIV may never be safe enough to use in a vaccine (see also "News Briefs," BETA, Summer 1999).
Studies done in 1991 with whole-killed SIV revealed that the protective immunity seen in macaques was due to antibodies directed against the cells in which the vaccine virus was cultivated and not the virus itself. This discouraging study derailed most whole-killed vaccine research. Immune Response Corporation, which developed Remune (made from a whole killed, gp120-depleted virus), is among the few companies still exploring this technology.
Researchers have noted that many structures displayed by this infection process would make excellent targets for neutralizing antibodies. The chemokine receptor site, which remains occult (hidden) until gp120 binds to the CD4 site, is one such target. Antibodies could also prevent cell fusion by binding to different parts of the transitional gp41 apparatus. Since these targets appear only fleetingly during viral infection, however, the means of utilizing them remained unknown. Jack Nunberg, Ph.D, and colleagues from the University of Montana, who published their findings in the January 15, 1999 issue of Science, may have solved this problem. Dr. Nunberg's team prepared a mixture of mouse cells displaying HIV Env proteins and human cells expressing both the CD4 and chemokine receptors. Formalin (a 40% solution of formaldehyde preservative) was added before the mouse and human cells fused together, thus freezing their activity -- and presumably capturing the Env proteins in a "fusion-competent" state.
To the surprise of many scientists, this idea worked. When the mixture (without formalin) was injected into mice, their antibodies not only could neutralize HIV, but they also were able to neutralize 23 of 24 different strains of HIV isolates taken from persons living in various regions of the world. This broad neutralization ability, which is essential for an HIV vaccine, had never been seen before. Dr. Nunberg's technique will no doubt serve as a model for future vaccine efforts; experiments in larger animals such as macaques are likely to ensue.
Initial, preliminary studies are quite promising. In a recent article published in the Proceedings of the National Academy of Sciences, Dr. David I. Cohen of Queens College in Flushing, NY, and colleagues suggest that the activity of Tat may explain why almost all vaccine candidates to date have failed. Dr. Cohen's team claims that every prophylactic vaccine strategy must take into account Tat's immediate assault on the immune system, because Tat will destroy immune responses brought on by any other part of the virus, such as envelope proteins.
Although Tat toxoid injections have not been able to prevent SIV infection in macaques, results of a phase IItrial of 30 HIV positive subjects (presented by Alessandro Gringeri, M.D., of the University of Milan, Italy) indicated that an experimental Tat vaccine was safe and able to induce a humoral immune response in all participants after seven injections. A phase III protocol based on this study has been developed. In the September 17, 1999 issue of AIDS Treatment News, Robert Gallo, M.D., Director of the Institute of Human Virology, announced that combining Tat toxoid with a vaccine against interferon-alpha, a type of cytokine, stabilized CD4 cell counts in persons taking no other anti-HIV therapy. (An overproduction of interferon-alpha, which is commonly seen in HIV positive people, inhibits the growth of lymphocytes.)
To elucidate the benefits of this combination, large-scale trials will need to be done. Researchers believe that injections of Tat toxoid will be used primarily as a therapeutic vaccine (in concert with other anti-HIV therapies) and as a possible component of a prophylactic vaccine. Because it is inexpensive, easy to administer, and far less complicated than HAART regimens, a Tat vaccine may one day play an important role in combating AIDS throughout the developing world.
Is there a conflict of interest when vaccine trial researchers offer prevention services to trial subjects yet depend on witnessing HIV infections to determine if the vaccine works? Can there be an international standard for informed consent? Is it fair to use people from resource-poor nations in vaccine trials if a successful vaccine most likely will be used primarily for people in developed countries? And once a vaccine is ultimately developed, will it be affordable and accessible for all people?
Such questions do not have easy answers. According to the September 11, 1999 issue of the Lancet, the Joint United Nations Programme on HIV/AIDS (UNAIDS) has revised its universal ethical guidelines concerning HIV trials, despite the fact that consensus on many key issues could not be reached.
Private organizations are stepping in to fund research and foster an environment for vaccine development by expanding public/private collaboration and investment. The American Foundation for AIDS Research (amfAR) has awarded seed grant money to a number of vaccine researchers, including Dr. Jack Nunberg, whose amfAR funds enabled him to complete his breakthrough work on fusion competency. The International AIDS Vaccine Initiative (IAVI), established in 1996, works on a variety of fronts to speed development and ensure distribution of preventive HIV vaccines. In all of its projects, IAVI emphasizes the need for a global vaccine effort; for example, it encourages pharmaceutical and biotechnology companies to conduct multiple clinical trials in different populations, especially in developing nations, to test vaccine efficacy against all HIV clades.
Private donations to IAVI are escalating, most notably from the Bill and Melinda Gates Foundation (Bill Gates is cofounder of the Microsoft Corporation), which donated $25 million as a five-year grant to IAVI this year. And a group of volunteer advocates located across the U.S., known as the AIDS Vaccine Advocacy Coalition (AVAC), has been goading the pharmaceutical and biotechnology industries, the government, and others to focus more energy and resources on vaccine development.
Perhaps due to input from organizations like AVAC and IAVI, U.S. government support is slowly on the rise. In an address to the United Nations General Assembly on September 21, 1999, President Clinton announced his intention to host a conference on vaccine development to spur investment and interest in vaccines against AIDS and other diseases.
This announcement came on the heels of Clinton's June dedication of the Dale and Betty Bumpers Vaccine Research Center at the National Institutes of Health (NIH), whose mandate is to develop an HIV vaccine under the guidance of newly appointed director Gary Nabel, M.D., Ph.D. Anthony Fauci, M.D., director of NIAID, notes that 11% of the NIH's current $1.8 billion (i.e., $198 million) allocated for AIDS research is devoted to vaccine research.
And late last year, President Clinton pledged to increase the NIH's AIDS vaccine research spending by 33%, to $200 million. In March, Representative Nancy Pelosi (D-CA) introduced the Lifesaving Vaccine Technology Act (H.R. 1274), which would provide a 30% tax credit for research and development expenditures on vaccines for tuberculosis, malaria, and HIV/AIDS. On October 12, Senator John Kerry (D-MA) introduced an identical bill (S. 1718) in the Senate. (As this issue of BETA went to press, Congress had not yet voted on either measure.)
A certain degree of support has been shown even on the state level. In September, the state of California awarded a six-year, $7 million grant to the University of California at San Francisco, which will use the funds to launch a new AIDS research center; part of its program will focus on HIV vaccines.
A movement is also afoot to create an international vaccine purchase fund. Such a fund would stimulate vaccine research and development by guaranteeing that any successful vaccine would be purchased in sufficient quantities at a certain price to provide a favorable return for the developer. Pharmaceutical companies therefore would enjoy the freedom and flexibility to pursue their own lines of inquiry without relying on government funding or directives.
Nicholas Cheonis is Editorial Assistant of BETA.
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